なくてはいけない

なくてはいけない expresses obligation or necessity: you must do something, or it is required. It is more formal and stronger than 〜たい/〜ほうがいい. Use it to describe duties, rules, or things you are required to do. The form changes slightly depending on the part of speech: verbs take the negative ない form plus なくてはいけない; i-adjectives use the なくてはいけない form; na-adjectives and nouns use でなくてはいけない. In casual speech, people often say なくちゃいけない or じゃなきゃいけない, but なくてはいけない is more formal. Example contexts include school assignments, work tasks, rules, or personal obligations.